Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 425this indicator tells you that a valid carrier signal exists between the local and remotemodem and that data transmission is possible.If the CD indicator should go off during a communications session its lack ofillumination informs you that data transmission is no longer possible. The cause ofa lack of continuity between modems can range from noise on a line to the remotemodem losing power and you should simply redial whenever you notice that theCD indicator has become extinguished.CTSIndicatorIn comparison to the Courier 2400 note that the T1000 does not have a CDindicator. Instead, you can turn on the modem's speaker to hear the presence orabsence of a carrier tone or observe the CTS indicator status.The clear to send CTS) indicator is illuminated when the modem is ready toaccept data from the attached terminal device for modulation. If no carrier signal ispresent the modem will not modulate data and its CTS indicator will not beilluminated.OH IndicatorThe off hook OH) indicator is illuminated when the Courier 2400 has takencontrol of the telephone line. The illumination of this indicator denotes that themodem has successfully performed a function similar to a person picking up atelephone handset. The lack of illumination of the OH indicator denotes the failureof the modem to take control of the line, a problem usually resulting from someonepreviously disconnecting the modem from the telephone line. If this indicator doesnot illuminate when you issue a dialing command you should check the cable fromthe modem to the telephone wall jack to verify it is correctly connected.RD IndicatorThe receive data RD) indicator of the Courier modem will ¯ash when a data bit isreceived from the telephone line or when the modem is sending a result code to anattached terminal device. You can use the RD indicator to isolate several types ofcommunications problems. As an example, assume the RD indicator is ¯ashing butno data is being displayed on an attached personal computer. This would indicatethat either the cable from the modem to the computer is defective or the personalcomputer's serial port has failed. Although the T1000 modem has the same RDindicator, readers should note that on some modems this indicator is labeled RX.SD IndicatorThe send data SD) indicator functions in a reverse manner to the RD indicator,that is, the SD indicator ¯ashes each time a bit is sent to the Courier 2400 modemfrom an attached terminal device.You can use the SD indicator to verify that data is reaching the modem from anattached terminal device once a carrier detect signal is present. This can beaccomplished by pressing characters on the terminal keyboard and observing the